When producing a semiconductor device, there have been many cases wherein semiconductor crystal layers of different conductivity-type are formed on a semiconductor substrate to form a circuit. In some cases, a buried layer or other conductivity-type region is arranged at interface therebetween. For example in a case wherein a longitudinal p-n-p transistor is formed to a p-type semiconductor substrate, a buried layer is formed in order to prevent collector current from flowing toward the substrate.
FIG. 9 shows a cross sectional explanatory view of a semiconductor wherein a p-n-p transistor is formed on a p-type semiconductor substrate. In FIG. 9, numeral 1 represents a p-type semiconductor substrate, numeral 2 represents a n.sup.+ -type buried layer, numeral 3 represents a semiconductor crystal layer, numeral 4 represents a p.sup.+ -type collector region, numeral 5 represents a p.sup.+ -type region, and numeral 6 is an isolation for insulating between the elements. When the layer 3 is formed by epitaxial growth, an impurity region formed on the surface of the substrate 1 is diffused into the layer 3 to form the n.sup.+ - type buried layer 2. After the buried layer 2 is formed, p-type impurity is injected into the impurity region. This p-type impurity is diffused, when the layer 3 is formed, to form the p.sup.+ -type collector region 4. The p.sup.+ -type collector region 4 is connected to a collector electrode C via the p.sup.+ -type region 5. Symbols E and B represent an emitter electrode and a base electrode respectively. The buried layer 2 is intended for preventing collector current from flowing toward the substrate.
For forming a p.sup.+ -type layer on a n.sup.+ -type buried layer, another method with two-stage epitaxial growth is also employed. This method is explained with reference to FIG. 10. A first layer of n-type epitaxial growth layer 7 is formed on a p-type semiconductor substrate 1, As .sup.+ or the like is injected thereinto, and it is diffused to form a buried layer 2. A second layer of n-type epitaxial growth layer 8 is formed, then acceptor impurity such as B.sup.+ or the like is injected thereinto, and it is diffused to form a p.sup.+ - region.
In a conventional case wherein semiconductor crystal layers of different conductivity-type are thickly formed on a semiconductor substrate, diffusion of isolation for insulating between elements is deepened and spread laterally to occupy a large area. On the contrary of this problem, there exists a requirement for forming on the substrate semiconductor crystal layers of reverse conductivity-type to that of the substrate in different thickness at different regions.
When forming the buried layer in the conventional longitudinal transistor, a n.sup.+ (or p.sup.+)-type buried layer having a high impurity concentration is formed in order to prevent collector current from flowing toward the substrate. However there was a problem that low resistance can not be realized because width of p.sup.+ -region is narrowed in the following step wherein auto doping easily happens. Besides, because on the n.sup.+ -region, n.sup.+ impurity is much introduced into the p.sup.+ -region, PN junction of high concentration can not be realized, and because the p.sup.+ -region is narrowed, it is impossible to increase a withstand voltage of the p.sup.+ -region and the n.sup.+ -region.
In the case of the two-stage epitaxial growth, a junction of high concentration can be realized on the n.sup.+ -region. However, because epitaxial growth must be twice performed, producing cost rises.